Conventionally, a fish finder that displays shoal-of-fish information detected using an ultrasonic wave on a display, while displaying information on water bottom sediment (i.e., rocks, stones, sands, mud, etc.), is known. The fish finder transmits a pulse of the ultrasonic wave towards water bottom from the transducer, and analyzes a water-bottom echo of the transmission pulse to obtain the bottom sediment information. The information is often used to know habitats of bottom fish, shrimps, crabs, and rocks causing breakage of a fishnet, etc. The bottom sediment is also detected using a sonic depth finder, as a part of oceanographic investigations.
When water bottom has many rocks and stones, and the water bottom surface is rough, because a water-bottom echo reflected from relatively a wide area around a water bottom position directly below the transducer is received by the fish finder, a time length of water-bottom echoes will be longer. On the other hand, when the water bottom surface is covered with sand or mud and is flat, because the reflected water-bottom echo can be received only from a narrow area, the echo length will be shorter. Further, as the water-bottom depth is deeper, the water-bottom echo length will be longer.
Japanese Patent No. 3,088,557 and No. 3,450,661 disclose a method of determining bottom sediment. This method first obtains a time length while water-bottom echoes exceed a predetermined threshold level, and a time after a pulse is transmitted until the water-bottom echoes are received. The method then divides the time length by the time to determine the bottom sediment.
Further, in European Patent No. 0501 743, a method of determining bottom sediment based on a received signal of a sonic depth finder, as shown in FIG. 6 is disclosed. In FIG. 6, reference numeral 61 indicates a signal that is the transmission pulse directly received, 62 indicates a primary water-bottom echo, 62a indicates a leading portion of the primary water-bottom echo, 62b indicates a tail portion of the primary water-bottom echo, and 63 indicates a secondary water-bottom echo. The secondary water-bottom echo is a reflection from the water bottom after the primary water-bottom echo reflects in a water surface or a ship's bottom. European Patent No. 0501 743 discloses that an integrated value of the tail portion 62b can be used as an index of roughness (coarseness of water bottom surface), that an integrated value of the entire range of the secondary water-bottom echo 63 is an index of hardness (hardness of the water bottom surface), and that the bottom sediment can be determined based on these integrated values.
Although the integrated value of the tail portion 62b is an index of the roughness, it is also influenced by the hardness of the water bottom surface. Japanese Unexamined Patent Application Publication No. 2007-178125, which filed by the present assignee, discloses a method of reducing the above influence by normalizing the signal of the tail portion 62b with its maximum value.
U.S. Pat. No. 6,801,474 discloses a method in which a pulse with a constant pulse width is transmitted, and amplitude data of water-bottom echo signals, which is sampled at a predetermined interval, is re-sampled at a frequency defined by a function of a water-bottom depth and a transmitting pulse width. The re-sampled amplitude data is collected at all research locations of a target ocean area, and feature vectors of water-bottom echoes at each research location is calculated from the collected amplitude data, where the feature vector represents characteristics of a shape and a width of the water-bottom echo, which is constituted with many elements more than 100. Then, three main components by which each element of the feature vector is linearly combined are calculated for each research location by a principal-component analysis, to classify the bottom sediment at each research location into classes of 5 to 10 based on values of the three main components. Coefficient used for the linear combination is calculated from the feature vectors at all the research locations.
Further, ACUSTICA acta acustica, (the Federal Republic of Germany), 2000, Vol. 86, p. 830-837, and “IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING” (U.S.), December 2001, Vol. 39, No. 12, p. 2722-2725 disclose a method using a neural network for the bottom-sediment classification.
Assuming a water-bottom depth is D, an half angle of a directivity angle of a ultrasonic wave transmitted from and received by a fish finder is θ, a transmitting pulse width is τ, a speed of sound in the water is c, and a water-bottom echo length is W, the length W is expressed by the following equation:W=2D(1/cosθ−1)c+τ   (1)As seen from the equation, the water-bottom echo length W depends on the water-bottom depth D as well as the transmitting pulse width τ. However, because this is not taken into consideration in Japanese Patent No. 3,088,557 and No. 3,450,661 and European Patent No. 0501 743, an accuracy of bottom-sediment classification degrades for shallow water where influence of τ is large.
In U.S. Pat. No. 6,801,474, a pulse with a constant pulse width is transmitted regardless of the water-bottom depth. Disadvantages caused by this will be explained referring to FIG. 7A. Here, assuming a half angle of a directivity angle of the ultrasonic wave transmitted from the transducer is θ. Further, assuming a water bottom position directly below the transducer when the water-bottom depth is D is P1, and a water bottom position pointed by the directivity angle is P2. Further, assuming a water bottom position directly below the transducer when the water-bottom depth is 2 D is P3, and a water bottom position pointed by the directivity angle is P4. For simplicity, an influence of propagation loss in the water is ignored, and the water-bottom echo signals from P1, P2, P3, and P4 will be considered. Here, as shown in FIG. 7B, assuming a time difference between water-bottom echo signals E11 and E12 from the water bottom positions P1 and P2, respectively, is T, a time difference between water-bottom echo signals E13 and E14 from the water bottom positions P3 and P4, respectively, will be 2 T. A width of the water-bottom echo signals E11-E14 is τ. Thus, between a combined signal G11 of the water-bottom echo signals E11 and E12, and a combined signal G12 of the water-bottom echo signals E13 and E14, a geometric similarity of signal waveforms will be spoiled even if the water bottom sediment is the same.
The method disclosed in U.S. Pat. No. 6,801,474 samples the amplitude data of the water-bottom echo signals at the predetermined interval, and then re-samples the amplitude data at the frequency defined by the function of the water-bottom depth and the transmitting pulse width. Thus, the numbers of data of the amplitude data F11 and F12 after the re-sampling become equal even if the water-bottom depths differ. However, because the geometric similarity of waveforms for the combined signals G11 and G12 could be spoiled, the feature vectors calculated from the amplitude data F11 and F12 after the re-sampling will differ from each other. Thus, there is a problem that the classification results of the bottom sediments at research locations are dependent on the water-bottom depth.
Further, because the method disclosed in U.S. Pat. No. 6,801,474 classifies after all the amplitude data of the water-bottom echo of all the research locations are collected, it is not suitable for use in the fish finder, which requires a real-time bottom-sediment classification. Further, because the method merely performs the classification, but it does not determine the bottom sediment at the research locations, there is another problem that the bottom sediment must actually be checked, such as with an underwater camera, at representative research locations for each class.